1. Field of the Invention
This invention relates to a thin film transistor device with a lightly doped drain (called "LDD") structure, a method of manufacturing the same and a thin film transistor liquid crystal display device.
2. Discussion of the Background
A thin film transistor (called "a TFT") made of a polycrystalline silicon semiconductor layer (collectively called "a p-Si TFT") is now researched and developed in order to comply with requirements for the integration of driver circuits with a p-Si TFT array formed on a glass substrate of a liquid crystal display device.
It is well known that, similarly to a single crystalline silicon TFT, a breakdown or deterioration of the TFT characteristics takes place with a p-Si TFT when an excessively strong electric field is applied between the source and drain regions thereof. Such a phenomenon is mainly caused by the occurrence of hot carriers due to concentration of the electric field on a certain location in the vicinity of the drain region so that it becomes a more critical problem as the TFT becomes much finer in structure.
In order to solve those technical problems a lightly doped drain structure is effective as often used in a single crystalline TFT. Since, however, anisotropic etching, low temperature processes and so on are required for devices formed on a large glass plate, such as those used for a liquid crystal display, the single crystalline processes are not always directly applicable to the polycrystalline processes.
There have been proposed process techniques for forming an LDD structure TFT which are different from those for single crystalline processes. Aside wall forming technique, for example, by means of anodic oxidation of the gate electrode is disclosed in Japanese Unexamined Patent Publication Hei 6-104279. A method of making use of side etching at the gate electrode is also disclosed in Japanese Unexamined Patent Publication Hei 5-17523. A method of making use of doping in a slant direction is further disclosed in Japanese Unexamined Patent Publication Hei 7-307477.